Medical instruments, for example catheters, are introduced into the human body during interventional and diagnostic procedures for a variety of purposes. Electrophysiological procedures on the heart are a known instance. It is important during procedures of said kind to know the momentary position of the medical instrument.
A known method for locating the medical instrument therein provides for recording two X-ray images of the region at the intervention site that are recorded at different angles and both show the medical instrument, and for determining and, where applicable, displaying the three-dimensional position of the medical instrument through back projection. However, said type of locating based on fluoroscopy has the disadvantage that both the patient and attendant medical personnel will be exposed to a high radiation dose because the X-ray images have to be recorded very often to keep track of the medical instrument. Said method has nonetheless remained in use as it exhibits a high degree of accuracy in position determining.
To reduce the patient's and medical personnel's exposure to radiation, alternative systems have been proposed that are not based on fluoroscopy.
A first group of said type of locating systems provides for one or more microcoils on the tip of the medical instrument. Provided outside the body are at least three stationary coils via which an electromagnetic signal is sent. The received signal from which the medical instrument's position can be determined through determining the distances from the transmitter coils and triangulation is measured.
In a second group of locating systems, catheters normally employed for obtaining intracardial ECG signals are used for electrophysiology. Three roughly orthogonally arranged pairs of external electrodes to which defined alternating voltages are successively applied are therein adhesively attached to the patient's skin. Through measuring the voltage on the catheter it is possible to determine the impedance between it and the skin electrodes and thus make an approximate inference about the distance. Again through triangulation, the medical instrument's position can be determined from distances from all three pairs of electrodes.
However, the cited X-ray-free locating systems both have the disadvantage that locating accuracy is limited. In contrast to the fluoroscopic methods, where an accuracy in the submillimeter range can be achieved, only accuracies in the range of 0.5 to 5 mm are achieved with the electromagnetic coil-based methods; errors in the centimeter range can occur in the case of the impedance-measuring systems.
Apart from by the patient's movements, particularly in the case of impedance measurements, said locating errors are caused in both groups of locating systems by distortions in the electromagnetic or, as the case may be, electric field. With the first group, errors are produced mainly by conducting materials such as metals in the area surrounding the patient. Metal parts of said type are, for example, an X-ray tube and a detector of an additionally present X-ray system. Complex calibrating techniques have been proposed for compensating said distortions and hence for achieving greater accuracy.
In the second group, the distortion in the field is due to the differing conductivity of different types of tissue etc. (fat, blood, lung tissue, etc.). That results in a non-linear voltage drop between the skin electrodes. Alongside extremely difficult local calibrating by means of instruments having a plurality of electrodes at a known distance, the use of reference electrodes has been proposed for compensating said effect, with its being possible in the latter case only to determine relative distances.